1. Field of the Invention
Our invention relates to bakeware obtained by molding polyester blend compositions, and preferably food-grade-compliant, disposable bakeware. Furthermore, our invention relates to methods of manufacturing bakeware from these polyester-blend compositions.
2. Description of the Related Art
Thermoplastic bakeware products for use in conventional and microwave ovens are well known in the art. These bakeware products typically include polyesters of ethylene, propylene, and butylene, particularly in their partially crystallized form. Those materials can withstand the high temperatures required of bakeware products. In addition, those materials are safe for storing and serving food items, i.e., they are considered to be food-grade materials.
However, a common problem with conventional thermoplastic bakeware is that the bakeware can catch fire. The risk of fire is primarily due to the dripping and stringing of the bakeware article onto a heat source, e.g., an electric heating element or open flame, when the thermoplastic material reaches its melting point. Upon direct contact with a heat source, the melted plastic may ignite quickly. The stringing or dripping plastic then can act as a bridge for the fire, whereupon flames may consume the whole article.
One solution for such unintentional fires has been to produce plastic bakeware products with flame retardants and/or fillers. Such flame-retardants are well known and examples include antimony, phosphate, and halide containing compounds. The flame retardants may act through different mechanisms, including the release of free radicals to terminate gas ignition, conversion of flammable gases to non-flammable gases, and promotion of char formation, to name a few. The addition of fillers can increase the potency of some flame retardants. For example, talc or calcium phosphate may be used to augment the known flame retardant ammonium phosphate (See, Levchik, et al., Polymer Degradation and Stability, 54, Nos. 2-3, pp. 361-3 (1996)). Reinforcing fillers typically include substances in a fibrous or plate-like form, such as carbon, talc, mica, glass, asbestos, wollastonite, titanium dioxide, boron whiskers, steel, copper, and aluminum. On their own, these fillers strengthen the formed product, providing for stiff, durable articles that retain their shape under physical stresses. However, such fillers can make the material brittle and may cause fabrication difficulties in certain manufacturing processes.
Even with the above-discussed methods for fire prevention, the plastic bakeware products currently available on the market typically melt at temperatures above 425° F., which, when used in a conventional oven, may cause them to string or drip down onto the heat source and catch fire. Accordingly, an important step in reducing the risk of fire is preventing the stringing or dripping of such bakeware products.
Conventionally, the prevention of stringing and dripping onto a heat source has been achieved through the use of protective sheets, upon which the bakeware may rest when placed in a conventional oven. However, the advantages of food-grade-compliant plastic bakeware products that will reduce or inhibit dripping and stringing at temperatures above 425° F. are significant when one considers that consumers may forget to place their plastic bakeware products on the protective sheets. Problems also arise with ovens that have poor temperature control. In these ovens, the user may set the oven at a temperature well below the melting point of the plastic, but the oven may actually regulate the temperature much higher, causing the plastic to melt.
Therefore, our invention is directed to bakeware made from thermoplastics that avoid the above-mentioned problems by using a blend of polyester and phenoxy resins in the compositions. As discussed in more detail later, transesterification occurring between the polyester and phenoxy resins at high temperatures helps to suppress/reduce dripping and stringing of the plastic.
U.S. Pat. No. 4,370,438 (“the 438 patent”) discloses compositions comprising a blend of two different polyesters. The main objective of the invention described in that patent is to prevent transesterification from occurring in the product, specifically between the blended polyesters. In fact, the patent fails to recognize any benefits in allowing transesterification to occur. A latter portion of the '438 patent indicates that the polyester compositions may optionally include phenoxy resins. However, we note that the compositions described in that patent are non-food-grade inasmuch as they contain fillers, such as glass fibers or heavy-metal salts, which are not approved by the Food and Drug Administration (FDA) for use in contact with food. Accordingly, that patent also fails to recognize the benefits of blending polyester and phenoxy resins in bakeware articles.
Furthermore, the compositions suggested in the '438 patent contain reinforcing agents. In particular, all of the embodiments exemplified in the '438 patent use reinforcing fillers at 31 percent, by weight. While the fillers provide flame retardant qualities, they can also cause the material to be too brittle to use in some molding techniques, such as thermoforming.
U.S. Pat. No. 3,962,174 also discloses polyesters for use in non-food-grade compliant articles. We believe the subject matter described therein suffers from the same disadvantages as the compositions discussed in the '438 patent.
These shortcomings in the related art are further evidenced by the lack of thermoplastic bakeware products available that reduce/suppress dripping and stringing at the higher temperatures possible with our invention. Specifically, currently available thermoplastic bakeware is more susceptible to the risk of fire, and thus less safe than articles according to our invention.